The invention relates to materials based on inorganic fibres, and especially materials intended to form part of the composition of thermal and/or acoustic insulating products or of soil-less culture media. The invention relates more particularly to the industrial production of glass wool or of rock wool, generally obtained by so-called internal centrifuging fibre-forming techniques.
This type of technique has been, and at present still is, very widely employed for manufacturing fibres which can be generally described as "single-component" (that is to say, in the context of the invention, obtained from a single type of molten inorganic material). Very roughly speaking, internal centrifuging consists of introducing a thin stream of a given molten inorganic substance, of the glass composition type, into a centrifuge, also called fibre-forming dish, which rotates at high speed and is pierced at its periphery with a multitude of orifices through which the molten substance is thrown out in the form of filaments under the effect of centrifugal force. These filaments are next drawn into fibres and entrained towards a collector in order to form a fibre sheet therein, under the effect of an annular high temperature and high speed drawing stream, running along the fibre-forming dish.
To ensure that the fibres collected together form a material, a sizing composition is sprayed onto the fibres as they travel towards the receiver. This composition usually contains a thermosetting resin. The sheet of collected fibres is next subjected to a heat treatment in an oven to cure/crosslink the resin, and thus to obtain a fibrous product exhibiting the desired properties, such as dimensional stability, mechanical tensile strength, regain of thickness after compression or the choice within the densities and thicknesses of the products.
For further details on the technique, reference will be made advantageously, for example, to U.S. Pat. Nos. 4,451,276; 4,756,732; 4,759,785; 4,759,974; 5,176,729; 5,270,434; 5,340,903; and 5,277,706 incorporated herein by reference.
However, attempts have always been made to improve the quality of these products, especially in terms of insulating power, of "volume" or of ability to regain thickness after compression. These properties depend on a number of parameters including the density of the product, the size of the fibres of which it is made up, or the proportion and the distribution of the sizing composition.
An adaptation of the internal centrifuging technique described above was then developed, an adaptation consisting, again very roughly speaking, in feeding each of the orifices in the fibre-forming dish with two thin streams of glass of different chemical compositions. Since the glass compositions are chosen so as to exhibit different thermal expansion coefficients, the fibres obtained are referred to as bi-component fibres. These fibres, on cooling, exhibit great flexibility and a "curvilinear" appearance giving the final product a particularly lofty appearance, particularly "puffed out", with an excellent regain of thickness after compression. Any sizing operation then becomes superfluous or even incompatible with this new technique, which often calls for a heat treatment of the sheet of fibres, once formed on line, at at least 370.degree. C. up to 590.degree. C., temperatures which would degrade the sizing resin beyond its cure by polymerization/crosslinking.
For further details on this fibre-forming technique, starting from a number of inorganic compositions, reference may be made to U.S. Pat. No. 2,998,620 or to the more recent U.S. patent application Ser. Nos. 275,184 filed Jul. 14, 1994 and 148,771 filed Nov. 5, 1993, both of which are incorporated herein by reference.
Furthermore, the fibrous products based on these bi-component fibres exhibit some disadvantages: the very method of obtaining these very special fibres can, in the present state of knowledge, produce only products of low use density, for example of the order of 8 to 10 kg/m.sup.3. Now, many applications, both in the field of thermal insulation and of acoustic insulation, require much denser fibrous materials reaching, for example, up to 100 to 110 kg/m.sup.3. The other significant disadvantage which needs to be stressed is the fact that, in this case, the geometrical shape of the sheet of fibres which is collected at the exit of the manufacturing line is only very poorly controlled: no dimensional stability is guaranteed, especially insofar as the thickness of the product is concerned. On an industrial scale, the only solution is, in fact, to package these products with the aid of a flexible plastic enclosure, a costly operation and one which is not entirely satisfactory since, while it allows the product to be handled more easily, it does not overcome the problem of the absence of control of the geometry of the final product.
The aim of the present invention is therefore to overcome all these disadvantages, especially by developing a fibrous material which can successfully reconcile the advantages of the fibrous materials based on "single-component" fibres with those of the fibrous materials based on bi-component fibres.